Scalable Multi-Channel Ranging

ABSTRACT

Techniques are presented herein to coordinate ranging exchanges between base stations in order to enable any number of wireless devices in the vicinity of the base stations to obtain signals associated with ranging exchanges between base stations, time-of-departure report messages transmitted by the base stations to each other and time-of-arrival report messages transmitted by the base stations to each other, for use in computing the location of the wireless devices. Based on the multi-channel time-of-arrivals computed for the wireless device with respect to each base station, the multi-channel time-of-arrivals contained in the time-of-arrival report messages transmitted between base stations and the known locations of the base stations, a physical location is computed for the wireless device.

TECHNICAL FIELD

The present disclosure relates to wireless communication networks anddevices.

BACKGROUND

In wireless networks, such as Wi-Fi™ wireless local area networks, oneuseful application is to determine the location of a wireless device(e.g., a client device). Accuracy in the location determination isimportant. Moreover, it is desirable to accurately locate a clientdevice in a scalable manner and without undue complexity.

Time-based ranging techniques work best with a wider frequency bandwidthof the ranging signals. A wider bandwidth results in a narrower (moreaccurate) measurement time resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system level diagram of a wireless network including aplurality of access points that make ranging exchanges on multiplechannels to facilitate a multi-channel or wideband location computationfor client devices.

FIG. 2 is a flow diagram generally depicting the operations performed bythe access points and client devices in the process presented herein.

FIGS. 3-5 are flow diagrams that illustrate in more detail theoperations performed by the access points and client devices in theprocess presented herein.

FIG. 6 is a flow chart generally depicting the operations performed in aclient device to compute its location by snooping ranging exchangesbetween access points.

FIG. 7 is a block diagram of an access point configured to participatein the process presented herein.

FIG. 8 is a block diagram of a client configured to participate in theprocess presented herein.

DESCRIPTION OF EXAMPLE EMBODIMENTS Overview

Techniques are presented herein to facilitate the computation of alocation of a wireless device (e.g., a client device) based on signalsexchanged between other wireless devices (generically referred to asbase stations, or more specifically access points) which are at knownlocations. The wireless device whose location is to be determinedreceives on each of multiple channels, ranging signals associated withranging exchanges between the base stations and stores samples ofreceived ranging signals associated with the ranging exchanges at thewireless device. The wireless device captures time-of-departure reportmessages transmitted between base stations. Each time-of-departurereport message includes information indicating, for the base stationthat is the source of the time-of-departure report message:single-channel time-of-departures for ranging signals transmitted by thebase station on respective ones of the multiple channels on which theranging signals were each sent and the destination base station to whicheach ranging signal was sent. The wireless device computes multi-channelwireless channel responses between the wireless device and each of thebase stations based on the stored samples of received ranging signals atthe wireless device on each of the multiple channels and thetime-of-departure report messages captured from the base stations. Thewireless device computes multi-channel time-of-arrivals between thewireless device and each of the base stations based on the multi-channelwireless channel responses computed for the wireless device. Thewireless device captures time-of-arrival report messages transmittedbetween base stations, each time-of-arrival report message includinginformation representing multi-channel time-of-arrivals from receivedranging signals during ranging exchanges on the multiple channels. Basedon the multi-channel time-of-arrivals computed for the wireless devicewith respect to each base station, the multi-channel time-of-arrivalscontained in the time-of-arrival report messages transmitted betweenbase stations and the known locations of the base stations, a physicallocation is computed for the wireless device.

A process is also presented herein to coordinate the ranging exchangesbetween base stations in order to enable any number of wireless devicesin the vicinity of the base stations to obtain signals associated withranging exchanges between base stations, time-of-departure reportmessages and time-of-arrival report messages, for use in computing thelocation of the wireless devices.

Example Embodiments

Presented herein are techniques to facilitate highly accurate locationdetermination of a wireless device based on signals exchanged betweenneighboring wireless devices. The techniques are described herein inconnection with a wireless local area network, in which the wirelessdevice whose location is to be determined is a wireless client device(also referred to herein as a “client”) and the neighboring wirelessdevices are wireless access points (APs). This is only one example, andit should be understood that these techniques are applicable to anywireless network in which a wireless device can monitor signalsexchanged between other wireless devices (e.g., more generally referredto as base stations and in other contexts as anchor nodes) that are atknown locations. Moreover, these techniques are applicable todetermining the location of base stations, e.g., APs as well.

Referring first to FIG. 1, a block diagram is shown of a wirelessnetwork 10 comprising a plurality of APs and clients. In the simplifiedexample of FIG. 1, there are three APs 20(1), 20(2) and 20(3), alsodenoted “AP A”, “AP B” and “AP C,” respectively, and two clients 30(1)and 30(2), also denoted “C1” and “C2,” respectively. In the example ofFIG. 1, the clients 30(1) and 30(2) are within range of all of the APs20(1)-20(3). In an actual network deployment, there may be numerous APs,and a given client may be within range of many more APs. Each of the APs20(1)-20(3) may be controlled by a wireless local area network (WLAN)controller 40 to which they communicate via a wired local area network(e.g. mesh network) shown at reference numeral 45.

According to the techniques presented herein, the APs (or AP moduleswithin other devices) switch radio frequency (RF) channels and exchangeranging packets with each other on each channel of a plurality ofchannels that span a frequency bandwidth. A ranging exchange is shown inFIG. 1 at reference numeral 50 between APs 20(1) and 20(2), at referencenumeral 52 between APs 20(1) and 20(3) and at reference numeral 54between APs 20(2) and 20(3). The APs 20(1)-20(3) publish a list ofsingle channel (SC) time-of-departures of signals in the rangingexchanges, compute multi-channel time-of-arrivals from the rangingexchanges on the multiple channels, and publish the multi-channel (MC)time-of-arrivals.

A client (any client desiring to compute its location), e.g., client30(1), follows in lock-step across the channels on which the rangingexchanges are made, capturing AP-to-AP ranging packets, and snooping SCtime-of-departure reports sent between the APs typically at the end ofthe exchanges. The client computes MC time-of-arrivals with respect toits reception of the AP-to-AP ranging packets, and uses AP locations andAP-published time-of-arrival reports to compute its own location via amulti-lateration computation.

There is also a network management device 47 that performs overallnetwork management of a wireless network. The network management device47 may participate in coordinating the location procedures, e.g.,scheduling of the ranging exchanges by the APs. In addition, the clientdevice and or APs, may offload to the network management device 47 thelocation related computations described herein.

Turning now to FIG. 2, a frequency bandwidth is spanned by a pluralityof channels 62. Each channel 62 may be a single unit of bandwidth or maybe comprised of multiple component channels such as those shown atreference numeral 64. Moreover, the channels 62 may have differentbandwidths. The multiple channels 62 may contiguously span aspectrum/bandwidth of interest, possibly partially overlapping. APsrendezvous on a sequence of channels, make ranging exchanges, send alist of SC time-of-departures (TODs) in report messages to each other,and send multi-channel time-of-arrival (TOA) report messages to eachother. As will become apparent from the following description, the TODreport messages include information indicating, for the AP that is thesource of the TOD report message: the TOD of each ranging packet/signaltransmitted by that AP, and channel identification informationindicating the channel on which the ranging packet/signal was sent andinformation indicating the destination AP to which the ranging signalwas sent. The MC TOA report messages transmitted between APs at 72include information representing a MC TOA synthesized from the rangingexchanges and lists of SC TODs on the multiple channels.

The schedule and identification of channels to which the APs switch toperform the operations noted in FIG. 2 is generallyidentified/represented at reference numeral 70. This information iscommunicated to clients so that each client can move from channel tochannel with the APs, and record samples for ranging packets received atthe client from the ranging exchanges between APs. As well, the clientcaptures the TOD report messages, and at 74 captures the MC TOA reportmessages sent between APs at 72, and ultimately computes its location orexports this information to another device where the client's locationis computed from this information and the location is sent back to theclient device or to some other destination. Multiple clients can performthis process in parallel and without any impact or influence on theprocessing performed by other clients. For simplicity, FIG. 2 (as wellas subsequent figures) shows only a single client involved, but this isnot meant to be limiting. In fact, it is one of several advantages ofthe techniques presented herein that numerous clients can be using theranging exchanges between APs and related TOD and TOA reporting toindependently determine or derive their locations.

Any of the APs may be configured to generate the schedule and channelidentification information 70. Alternatively, the WLAN controller 45(FIG. 1) may be configured to generate the schedule and channelidentification information 70. In either case, each of the APs is madeaware of the schedule and channel information either by way of aninter-AP message or a network management message received from the WLANcontroller. Moreover, any client that is to participate in the locationscheme is sent the schedule and channel information 70. For example,each AP, when it receives the schedule and channel information, maybroadcast the schedule and channel information 70 in an appropriatenetwork management message for all clients. In this way, any client thatis currently being served by an AP will ultimately receive the scheduleand channel information and be able to follow the APs as they switchfrom one channel to another. In one example, the ranging exchangeconsists of a Fine Timing Measurement frame (or other frame such as avendor specific frame) followed by an acknowledgement (Ack) or Block Ackframe in accordance with the IEEE 802.11 communication standard(specifically 802.11mc Draft 1.0). However, for applications in3G/4G/LTE cellular systems, the ranging exchanges would take differentforms, which would be known to those of ordinary skill in the art.

FIGS. 3-5 show the process is more detail. Referring first to FIG. 3, adiagram is shown of operations performed by APs that are in-range ofeach other. The APs may perform this process on-demand, or when theyexpect clients to be using the sequence of exchanges, when they detectclients that are capable of using the sequence of exchanges, or always.Again, the example of three APs is used, but this is not meant to belimiting. The APs (or a module within an AP box) are coarselysynchronized (e.g., at a millisecond scale). Every period, e.g. every Nbeacon intervals, N>=1 or every few seconds, at a scheduled time, eachof these APs rendezvous at a common channel and performs a rangingexchange with each of its neighbors.

In the example of FIG. 3, the APs A, B and C initially rendezvous onChannel A. Channel A is, for example, a frequency bandwidth consistingof Channels 36+40+44+48 in the 5 GHz frequency band used by IEEE802.11a/n/ac. In Channel A, the three APs engage in ranging exchangesshown at reference numerals 50(A), 52(A) and 54(A). A ranging exchangeis, in the case of ranging exchange 50(A), for example, a packet from APA to AP B then a response (e.g., Ack or Block Ack) from AP B to AP A.The other ranging exchanges 52(A) and 54(A) may be similar to that ofranging exchange 50(A).

The sequence of a ranging exchange is characterized by 4 timestamps: TODof a packet from AP A (t1), TOA of that packet at AP B (t2), TOD of aresponse from AP B (t3), and TOA of that response at AP A (t4). Theassociated timestamps of each exchange that need to be transmitted aret1 and t4 (from AP A).

During these ranging exchanges (at different times on differentchannels), each participating AP records the TOD of any packet/signal itsends and the in-phase (I) and quadrature (Q) samples together with atimestamp value of the first recorded sample associated with receptionof a ranging signal/packet from the other AP (or some portion thereofsuch as a portion of the preamble of the packet). The IQ data may berecorded in the time or frequency domain, before or after datamodulation is removed. Once an AP has the list of SC TODs from a peerAP, the AP generates and stores a wireless channel estimate (impulseresponse) between itself and the peer AP from all received AP-to-peer-APranging packets and the SC TODs. The AP also generates and stores areference time for the exchange, such as the first processed sample ofthe first AP-to-peer-AP exchange. The AP does this for each peer AP.Furthermore, the client generates and stores I and Q samples togetherwith a timestamp value of the first recorded sample associated withreception of ranging packets sent between APs during the rangingexchanges. From the stored I and Q samples, the client generates a MCchannel response estimate (impulse response) and a reference time forthe wireless channel between the client and each of the respective APs.In general, for N APs that the client is snooping ranging transmissionsfrom, the client records I and Q samples for N*(N−1)*#channels*2 APtransmissions. In the example described herein, N=3.

For example, in ranging exchange 50(A), AP A records the TOD of theoutgoing packet 80 to AP B, records the I and Q samples together with areference time for reception of the response packet 82 from AP B, orprocesses the I and Q samples and records a wireless channel responsetogether with a reference time for the reception of the response packet82 from AP B. Thus, in FIG. 3, “TOD” and “IQ” labels on the respectiveends of the arrows shown in FIG. 2 for the ranging exchanges 50(A),52(A) and 54(A) refer to the TOD of a packet/signal and the I and Qsamples of a received packet/signal, respectively. It should beunderstood that while not separately denoted in FIG. 3, the IQ samplesmight be replaced by the wireless channel response estimate for thereception of that packet; but either are paired with a reference time.Typically, there are 3-10 neighbors per AP, and the ranging exchange isvery short (e.g. 100 μs), such that these ranging exchanges consumebetween 1-10 milliseconds.

After an agreed time interval, all the APs rendezvous at the next commonchannel and repeat the ranging exchanges with each neighbor AP. Thistime interval is shown at 90 in FIG. 3 and serves as a guard timeinterval to account for any synchronization errors in the APs or clientswhen moving to the new channel Thus, in the example of FIG. 3, the APsnext move to Channel B, which consists of the 5 GHz Channels52+56+60+64. The ranging exchanges in Channel B are shown at referencenumerals 50(B), 52(B) and 54(B). After the ranging exchanges in ChannelB, the APs rendezvous in Channel C and perform ranging exchanges 50(C),52(C) and 54(C). Channel C may consist of Channels 100+104+108+112 inthe 5 GHz band. Note that the order of these ranging exchanges inChannel B need not be the same as in Channel A. The order of the rangingexchanges may randomly change from channel to channel due to contentionetc.

This process repeats until all common channels are visited. For example,at the high end this could be 160 MHz (from Channels 36-64) then 80 MHz(Channels 100-112) then additional 80 MHz channels (Channels 116-128,Channels 132-144, Channels 149-161). As shown in FIG. 3, clients areaware of the schedule of the AP-to-AP ranging exchanges and switchchannels with the APs in order to receive all AP-to-AP rangingtransmissions, store I and Q samples and generate channel responseinformation.

Reference is now made to FIG. 4. After all the ranging exchanges havebeen made on all the desired channels, in one embodiment the APs publishthe list of SC TOD information stored from the ranging exchanges. (Notethat the TOD information could be published for each channel beforeswitching to the next channel, or published on the next channel for theprevious channel and thus, the order of the processing phases shown inFIGS. 3-5 is not meant to be limiting.) The APs publish the TODinformation by sending TOD report messages to each other. In oneembodiment, AP A sends a TOD report message 100 to AP B and a TOD reportmessage 102 to AP C. AP C sends a TOD report message 104 to AP B and aTOD report message 106 to AP A. AP B sends a TOD report message 108 toAP C and a TOD report message 110 to AP A. Alternatively, each AP maysend its list of TOD information in a broadcast or multicast frame. EachTOD report message includes the TOD of the ranging signal transmitted bythat AP, channel identification information indicating the channel onwhich the ranging signal was sent and information indicating thedestination AP to which the ranging signal was sent and potentiallyother identifying information (such as a dialog token of the firstranging frame in an exchange). For example, TOD report messages 100 and102 include, for each ranging signal sent by AP A: the TOD of theranging signal, information indicating the channel on which that rangingsignal was sent by AP A, and information indicating to which AP thatranging signal was sent. Similarly, the TOD report messages 104 and 106sent by AP C include, for each ranging signal sent by AP C: the TOD ofthe ranging signal, information indicating the channel on which thatranging signal was sent by AP C, and information indicating to which APthat ranging signal was sent. Finally, TOD report messages 108 and 110sent by AP B include: the TOD of the ranging signal, informationindicating the channel on which that ranging signal was sent by AP B,and information indicating to which AP that ranging signal was sent.

As shown at reference numeral 120, the client device snoops (anddecodes) the TOD report messages sent by the APs, and stores theinformation contained in the TOD report messages. The TOD reportmessages (containing the list of SC TODs) are designed to assistsnooping, such as by being sent to a broadcast address, a well-known orlocally configured multicast address, and being sent with known parsablefields (e.g. an Action management frame with known Category and Actionfields).

Given the channel estimates recorded for reception of each rangingpacket and the SC TODs, each AP and the client can “stitch” together inthe frequency domain a wideband/multi-channel wireless channel estimate(impulse response) with respect to a nearby AP and compute a measure ofthe TOAs of the stitched multi-channel transmissions from themulti-channel wireless channel estimate (i.e., MC TOAs). Each APperforms this stitching computation and publishes a multi-channel TOA.This is complicated by clock offsets between APs and the client, whichcan be estimated in several ways (estimating the clock offset from thecarrier offset recognizing that devices have locked baseband and carrieroscillators, estimating clock drift across multiple exchanges, etc).Once estimated, the IQ samples may be resampled or the TODs/TOAs scaledaccording to the estimated clock offsets.

Specifically, as shown in FIG. 4, at 130, AP A computes a multi-channelwireless channel response between it and AP B and a multi-channelwireless channel response between it and AP C using (a) the IQ samplesor channel responses AP A stored for received ranging signals from APs Band C in each of the channels in which ranging exchanges were performedand (b) the lists of SC TODs (the TOD information received from APs Band C with respect to ranging signals sent by APs B and C during theranging exchanges in each of the multiple channels). Furthermore, usingthe multi-channel wireless channel responses (with respect to APs B andC), AP A computes multi-channel TOAs from these multi-channel wirelesschannel responses stitched together from the received transmissions fromAPs B and C in each of the multiple channels.

Similarly, at 132, AP B computes a multi-channel wireless channelresponse between it and AP A and a multi-channel wireless channelresponse between it and AP C using (a) the IQ samples or channelresponses AP B stored for received ranging signals from APs A and C ineach of the multiple channels in which ranging exchanges were performedand (b) the lists of SC TODs (the TOD information received from APs Aand C with respect to ranging signals sent by APs A and C during theranging exchanges in each of the multiple channels). Using themulti-channel wireless channel responses (with respect to APs A and C),AP B computes multi-channel TOAs from these multi-channel wirelesschannel responses stitched together from the received transmissions fromAPs A and C in each of the multiple channels.

At 134, AP C computes a multi-channel wireless channel response betweenit and AP A and a multi-channel wireless channel response between it andAP B using (a) the IQ samples or the channel responses AP C stored forreceived ranging signals from APs A and B in each of the multiplechannels in which ranging exchanges were performed and (b) the lists ofSC TODs (the TOD information received from APs A and B with respect toranging signals sent by APs A and B during the ranging exchanges in eachof the multiple channels). Using the multi-channel wireless channelresponses (with respect to APs A and B), AP C computes multi-channelTOAs from these multi-channel wireless channel responses stitchedtogether from the received transmissions from APs A and B in each of themultiple channels.

Like the computations made by APs A, B and C, at 136, the clientcomputes a multi-channel wireless channel response between it and AP A,a multi-channel wireless channel response between it and AP B, and amulti-channel wireless channel response between it and AP C using the IQsamples or channel responses the client stored for received rangingsignals from APs A, B and C in each of the multiple channels in whichranging exchanges were performed and the TOD information snooped fromthe TOD report messages published by APs A, B and C to each other withrespect to ranging signals sent by APs A, B and C during the rangingexchanges in each of the multiple channels. Using the multi-channelwireless channel responses (with respect to APs A, B and C,respectively), the client computes multi-channel TOAs from thesemulti-channel wireless channel responses stitched together from thereceived transmissions from APs A, B and C in each of the multiplechannels.

The techniques to derive a multi-channel TOA from the ranging exchangeson the individual channels spanning the wideband frequency bandwidth isgenerally known in the art. However, a general description follows. Thereceiver (at each AP and the client device) digitally mixes thesingle-channel channel impulse responses to their known offsetfrequencies and sums them to produce a wideband channel impulseresponse. In an alternative embodiment the channel transfer functionsare concatenated in the frequency domain (with zero-padding for edgetones where the channel transfer function (CTF) is unknown) andtransformed to the time domain. In either embodiment, the multi-channelchannel input response is processed to discern a first impulse. In afurther embodiment, the raw IQ samples (i.e., with data modulation) ofthe known portion of the packets from each channel are mixed to theirknown offset frequencies and summed to compute a composite multi-channelmeasured signal. As well, the receiver computes a compositemulti-channel reference signal corresponding to the known portion of theindividual received single-channel waveforms (without accounting formultipath effects) mixed to their appropriate channel offset. Thecomposite multi-channel measured signal is cross-correlated with thecomposite multi-channel reference signal (to identify the multipathchannel), then the first-impulse is detected.

Reference is now made to FIG. 5 for the final phase of the process. At140, AP A sends a TOA report message to AP B and at 142 sends a TOAreport message to AP C. The TOA report message sent by AP A to AP Bincludes information representing the multi-channel TOA for rangingpackets AP A received from AP B. Similarly, the TOA report message sentby AP A to AP C includes information representing the multi-channel TOAfor ranging packets AP A received from AP C. At 144, AP B sends a TOAreport message to AP A and at 146 AP B sends a TOA report message to APC. The TOA report message sent by AP B to AP A includes informationrepresenting the multi-channel TOA for ranging packets AP B receivedfrom AP A. Similarly, the TOA report message sent by AP B to AP Cincludes information representing the multi-channel TOA for rangingpackets AP B received from AP C. At 148, AP C sends a TOA report messageto AP B and at 150 AP C sends a TOA report message to AP A. The TOAreport message sent by AP C to AP B includes information representingthe multi-channel TOA for ranging packets AP C received from AP B.Similarly, the TOA report message sent by AP C to AP A includesinformation representing the multi-channel TOA for ranging packets AP Creceived from AP A.

As shown at 160, the client snoops the TOA report messages sent betweenAPs in order to capture the TOA information contained in those messages.Using the MC TOA information that the client obtained from the TOAreport messages, at 170, a precise location for the client may becomputed based further on the multi-channel TOA of multi-channel rangingtransmissions stitched together from the received transmissions from APsA, B and C in each of the multiple channels and in turn on the TODinformation associated with the inter-AP ranging transmissions containedin the TOD report messages that the client snooped at 120 in FIG. 4.More specifically, given the AP locations (which are known and can besent in a management message to the client in advance), the TODs in theTOD report messages and the multi-channel TOA information in the TOAreport messages, and its own estimated multi-channel TOA informationwith respect to ranging transmissions from nearby APs, there issufficient information to calculate time difference of arrivals (TDOAs)between the client and nearby APs and then calculate its own locationfrom the TDOAs. The TOA report messages are designed to assist snooping,such as by being sent to the broadcast address, a well-known or locallyconfigured multicast address, and being sent with known parsable fields(e.g. an Action management frame with known Category and Action fields).

The computations made at operation 170 are based on location computationtechniques known in the art for transmissions made in a single frequencychannel. These same techniques may be used where the measurements madein a single frequency channel are replaced with the multi-channel TOAand related TODs, etc., as described above in connection with FIGS. 3-5.Consider the two-dimensional example where APs A, B and C are at knownlocations: (0,0), (0,b) and (c_x, c_y), respectively, and the client islocated at (x,y). The simplistic differential difference equations thatdo not account well for measurement noise are:

$R_{ab} = {\sqrt{x^{2} + y^{2}} - \sqrt{\left( {x - b} \right)^{2} + y^{2}}}$${R_{a\; c} = {\sqrt{x^{2} + y^{2}} - \sqrt{\left( {x - c_{x}} \right)^{2} + \left( {x - c_{y}} \right)^{2}}}},{where}$$c = \sqrt{c_{x} + c_{y}}$$d = {\frac{R_{ab}}{2} - \frac{b^{2}}{2R_{ab}}}$ $e = \frac{b}{R_{ab}}$$g = {\frac{R_{a\; c}b}{R_{ab}c_{y}} - \frac{c_{x}}{c_{y}}}$$h = \frac{c^{2} - R_{a\; c}^{2} + {R_{a\; c}{R_{ab}\left( {1 - \left( \frac{b}{R_{ab}} \right)^{2}} \right)}}}{2c_{y\;}}$

Two curves are defined:

$\quad\left\{ \begin{matrix}{y = {{gx} + h}} \\{y = {\pm \sqrt{{\left( {e^{2} - 1} \right)x^{2}} + {2{edx}} + d^{2}}}}\end{matrix} \right.$

The location (x,y) of the client lies on the intersection of the twocurves.

More sophisticated algorithms that account for multiple APs and/ormeasurement noise may be used. In one embodiment, first the area inwhich the client can be located is finely gridded. For each AP pair, a2-dimensional array called a “heatmap” is defined that contains thepredicted differential time of flight from a first AP to a second APwith respect to the client. The measured differential time-of-arrivalbetween the two APs is calculated as:

(MC_TOA_fromfirstAP_atClient−(MC_TOA_from_secondAP_atClient−(MC_TOA_from_secondAP_atFirstAP−first_SC_TOD_from_firstAP_toSecondAP−timeOfFlightBetweenFirstandSecondAp)).

A metric is computed based on the squared difference between the heatmap(the predicted differential time of flight) and the measureddifferential time-of-arrival between the two APs, summed across eachpair of APs, computed for each grid point. The result, E, can bevisualized as a map of elevations where the lowest elevation representsthe maximum likelihood location. Alternatively, the map of elevationscan be converted to a probability map, assuming Gaussian measurementnoise of variance σ_(n) ², via the operation exp(−E²/(2σ_(n) ²)) up toan unimportant scalar. By taking the center of mass of the probabilitymap, the client's minimum mean squared error (MMSE) location can bedetermined. As written, this is an O(N²) computation in the number ofAPs, but by rearranging terms the sum of squared errors across pairs ofAPs. this can be rearranged into an O(N) computation.

The following sets forth a mathematical description of the computationof a client device location using this heatmap technique.

APn transmits to APm at t1,mn (scalar), APm responds and is heard at APnat t4,mn (scalar). The time of flight between APm and APn is Tmn(scalar), between APm and client is Tmc (scalar), and between APn andclient is Tnc (scalar). Client receives (hears) APm's and APn'stransmissions at Umc and Unc, respectively (scalars).

A 2-dimensional (2D) array Hm called a heatmap is defined. Each entry ofHm corresponds to an XY location on an even grid. Each entry of Hm isthe elapsed time for light to travel from APm to that entry'scorresponding XY point—like Tmc if the client c were moved to each gridpoint in turn. Hn (2D array) is the time of flight from AP to gridpoints. Hmn=Hm-Hn (2D array). Hmn=Hm-Hn is the “reference”.

The measurement from APn is Unc=t1+Tnc so Tnc=Unc−t1. The measurementfrom APm is Umc=t4−Tmn+Tmc so Tmc=Umc−t4+Tmn. Combining,Tmc−Tnc=Umc−Unc+t1,mn−t4,mn+Tmn (scalar).

Define t1,mn−t4,mn+Tmn=Xmn (scalar), this can be rewritten asTmc−Tnc=Umc−Unc+Xmn (scalar).

The aforementioned metric is

M=SUM_m=1:nAP {SUM n=1:nAp (Hmn−(Umc−Unc+Xmn))²}, which is a 2D arraybecause Hmn is 2D. This is an O(nAp²) calculation of the 2D Hmn array.

Expanding and rearranging,

M = SUM_m = 1:nAP{SUM  n = 1:nAP((Hm − Umc) − (Hn − Unc) + Xmn)²} = nAp * SUM_m = 1:nAP(Hm − Umc)² + nAp * SUM_n = 1:nAP(Hn − Unc)² + SUM_m = 1:nAP{SUM  n = 1:nApXmn²} − 2 * SUM_m = 1:nAP(Hm − Umc) * SUM_n = 1:nAP(Hn − Unc) + 2 * SUM_m = 1:nAP{(Hm − Umc) * SUM_n = 1:nAPXmn} + 2 * SUM_n = 1:nAP{(Hn − Unc) * SUM_m = 1:nAPXmn}

Note SUM n=1:nAp Xmn², SUM_m=1:nAP Xmn and SUM_n=1:nAP Xmn are scalarterms that can be computed very quickly. The summations over theheatmaps Hm and Hn are now only O(nAp) computations, providingsignificant complexity reduction.

As described above in connection with FIGS. 3-5, in one embodiment, theclient may make all the necessary channel response estimatecomputations, multi-channel channel response computations, multi-channelTOA computations and ultimately location computations to determine itslocation from the information snooped from the inter-AP transmissions.In another embodiment, the client may offload the raw data it generatesand obtains from snooping the inter-AP transmissions to another device,e.g., a network management server with location computation capability,so that the computations are made by another device on behalf of theclient and the location computation result is sent back to the clientdevice, or to some other destination.

Turning to FIG. 6, a flow chart is provided that summarizes theoperations performed to compute a location of a wireless device, e.g., aclient device, according to the techniques presented herein. At 200, theclient device receives on each of multiple channels signals associatedwith ranging exchanges between the APs and stores samples of receivedsignals at the client device associated with the ranging exchanges. At210, TOD report messages transmitted between APs are captured at theclient device. Each TOD report message includes information indicating,for the AP that is the source of the TOD report message: single-channelTODs for ranging signals transmitted by the AP on respective ones of themultiple channels, and the channel on which the ranging signals wereeach sent and the destination AP to which each ranging signal was sent.As explained above, the TOD report messages may also include anidentification of the frame transmitted by the respective APs during theranging exchanges, the frame identification possibly being a dialogtoken. At 220, the client device computes a multi-channel wirelesschannel response between it and each of the APs based on the storedsamples of received signals at the client device on each of the multiplechannels and the TOD report messages captured from the APs. Thecomputation of the multi-channel wireless channel response may involvecorrecting the TODs contained in the TOD report messages for clockoffsets between the APs. At 230, the client device computesmulti-channel TOAs between it and each of the APs based on themulti-channel wireless channel response computed for the client device.At 240, the client device captures TOA report messages transmittedbetween APs. Each TOA report message includes information representingmulti-channel TOAs from received ranging signals during rangingexchanges on the multiple channels. At 250, a location for the clientdevice is computed based on the multi-channel TOAs computed for theclient device with respect to each AP, the multi-channel TOAs containedin the TOA report messages transmitted between APs and the knownlocations of the APs. The location computation at 250 may involvecorrecting for clock offsets between the APs.

In a further embodiment, each AP-to-AP transmission per channel is sentvia multiple antennas (e.g., sequentially switched through the APantennas) with known modulated data across the signal bandwidth for eachantenna. This enables the client device to compute the wireless channelresponse from each AP antenna. The client device can therefore computethe full multiple-input multiple-output (MIMO) multi-(frequency) channelwireless channel response with respect to each AP. The wireless devicealso computes a one-dimensional angle of arrival (typically azimuth) ora two-dimensional angle of arrival (typically azimuth and elevation,though this may be mapped to the XY plane instead) for the MIMO wirelesschannel impulse response at a first path of a multipath channel impulseresponse (i.e. the angle of arrival of the line-of-sight path). Theangle of arrival may be represented as a probability distribution ofangle-of-arrival (or XY position) instead of a single number. Thus, forthis further embodiment in which MIMO capture is used, the client devicemay additionally use the angle of arrival information when computing itsphysical location. In summary, the wireless client device computes afull MIMO multi-channel wireless channel response with respect to eachof the plurality of APs (base stations), computes an angle of arrivalfor a MIMO wireless channel response at a first path of a multipathchannel response, and computes its location based further on the angleof arrival.

FIGS. 7 and 8 illustrate examples of block diagrams of an AP and aclient device configured to perform the operations presented herein.With reference first to FIG. 7, there are shown relevant components ofan AP at reference numeral 20(i). Reference numeral 20(i) is used togenerically refer to any of the APs shown in FIGS. 1-6. The AP 20(i)includes one or more antennas 300, a transmitter (or bank oftransmitters) 310, a receiver (or bank of receivers 320), a basebandprocessor 330 (e.g., a modem), a controller 340, a network interfacecard 350, and memory 360. In the case where the AP includes multipleantennas 300, then there may be a corresponding number of transmittersand a corresponding number of receivers. The baseband processor 330performs baseband modulation and formatting of transmit signals that aresupplied to the transmitter for upconversion and transmission via theantennas 300. Similarly, the baseband processor 330 performs basebanddemodulating of signals received and downconverted by the receiver 320.To this end, the baseband processor 330 generates IQ samples and channelestimates from received signals, and supplies this data to thecontroller 340. In addition, the controller 340 may supply TODinformation for outgoing transmissions, e.g., ranging packets, to besent by the AP 20(i). Alternatively, the baseband processor 330 maygenerate the TOD information for outgoing transmissions. The NIC 350enables wired network, e.g., Ethernet, communications for the AP 20(i).

The controller 340 may be a microprocessor or microcontroller, andexecutes software instructions stored in memory 360. The memory 360stores multi-channel location coordination software 370 that, whenexecuted by the controller 340, cause the controller 340 to performcontrol the operations of the baseband processor 330 to perform theoperations AP described above in connection with FIGS. 2-5.

Turning to FIG. 8, the block diagram of a client device is nowdescribed. The client device, shown generically at reference numeral30(i), includes an antenna 400 (or multiple antennas), a transmitter410, a receiver 420, a baseband processor (e.g., modem) 430, acontroller 440 and memory 450. The baseband processor 430 performsbaseband modulation and formatting of transmit signals that are suppliedto the transmitter 410 for upconversion and transmission via the antenna400. Similarly, the baseband processor 430 performs basebanddemodulating of signals received and downconverted by the receiver 420.To this end, the baseband processor 330 generates IQ samples and channelestimates from received signals, and supplies this data to thecontroller 440. The memory 450 stores software instructions formulti-channel location software 460 that, when executed by thecontroller 440, cause the controller 440 to perform the clientoperations described above in connection with FIGS. 2-5.

For example, the controller 450 controls the receiver (via the basebandprocessor 430 or directly) to receive ranging exchanges between APs onone channel, wait a period of time, and thereafter receive rangingexchanges between APs on another channel. This reception control isbased on the received schedule and identification information of thechannels in which the ranging exchanges between APs are to be made, andas described above, is also used to capture time-of-departure reportmessages and time-of-arrival report messages transmitted between APs.

Memory 360 and 450 shown in FIGS. 7 and 8 may comprise read only memory(ROM), random access memory (RAM), magnetic disk storage media devices,optical storage media devices, flash memory devices, electrical,optical, or other physical/tangible memory storage devices. Thus, ingeneral, the memory 360 and 450 may comprise one or more tangible(non-transitory) computer readable storage media (e.g., a memory device)encoded with software comprising computer executable instructions andwhen the software is executed (by a processor) it is operable to performthe operations described herein

In summary, techniques are presented herein in which APs (or AP modules)switch channels, exchanging ranging packets on each channel, computingmulti-channel TOAs, and publishing the TODs and time-of-arrivals.Clients follow in lock step across the channels, capturing AP-to-APranging packets, snooping TODs sent by APs, and compute TOAs of theAP-to-AP ranging packets. Using knowledge of the AP locations andAP-published TOAs, the location of the clients can be computed usingvarious computations, such as multi-lateration.

The techniques presented herein result in a highly accurate locationdetermination with minimal overhead, and are highly scalable acrossmultiple client devices simultaneously. There is no need to take one ormore APs off-channel to locate one client, as many current locationschemes require. Once clients have the AP locations (and maps and pointsof interest, which could be downloaded by clients ahead of time) thereis no client traffic needed for the clients to determine theirlocations. This is very advantageous for low latency turn-by-turnnavigation in stadiums, conference venues, higher education venues, etc.Moreover, by synthesizing a wide bandwidth set of data from rangingexchanges in multiple channels that span a wide bandwidth, higherlocation accuracy can be achieved from the resulting TOD and TOA data.

Thus, in summary, from the perspective of a wireless device whoselocation is to be computed, a method is provided comprising: at awireless device operating in a wireless network that includes aplurality of wireless base stations at known locations, receiving oneach of multiple channels, ranging signals associated with rangingexchanges between the base stations and storing at the wireless devicesamples of received ranging signals associated with the rangingexchanges; capturing time-of-departure report messages transmittedbetween base stations, each time-of-departure report message includinginformation indicating, for the base station that is the source of thetime-of-departure report message: single-channel time-of-departures forranging signals transmitted by the base station on respective ones ofthe multiple channels on which the ranging signals were each sent andthe destination base station to which each ranging signal was sent;computing multi-channel wireless channel responses between the wirelessdevice and each of the base stations based on the stored samples ofreceived ranging signals at the wireless device on each of the multiplechannels and the time-of-departure report messages captured from thebase stations; computing multi-channel time-of-arrivals between thewireless device and each of the base stations based on the multi-channelwireless channel responses computed for the wireless device; capturingtime-of-arrival report messages transmitted between base stations, eachtime-of-arrival report message including information representingmulti-channel time-of-arrivals from received ranging signals duringranging exchanges on the multiple channels; and computing a location forthe wireless device based on the multi-channel time-of-arrivals computedfor the wireless device with respect to each base station, themulti-channel time-of-arrivals contained in the time-of-arrival reportmessages transmitted between base stations and the known locations ofthe base stations.

Similarly, from the perspective of a wireless device whose location isto be computed, an apparatus is provided comprising: a receiverconfigured to receive wireless signals including signals transmittedbetween base stations as part of ranging exchanges between the basestations; a memory configured to store data derived from reception ofwireless signals by the receiver; and a controller coupled to thereceiver. The controller is configured to: control the receiver toreceive on each of multiple channels, ranging signals associated withranging exchanges between the base stations; store in the memory samplesof received ranging signals associated with the ranging exchanges;extract from received time-of-departure report messages transmittedbetween base stations, information indicating, for the base station thatis the source of the time-of-departure report message: single-channeltime-of-departures for ranging signals transmitted by the base stationon respective ones of the multiple channels the channel on which theranging signals were each sent and the destination base station to whicheach ranging signal was sent; compute multi-channel wireless channelresponses with respect to each of the base stations based on the storedsamples of received ranging signals on each of the multiple channels andthe time-of-departure report messages; compute multi-channeltime-of-arrivals with respect to each of the base stations based on themulti-channel wireless channel responses; extract from receivedtime-of-arrival report messages transmitted between base stationsinformation representing multi-channel time-of-arrivals from receivedranging signals during ranging exchanges on the multiple channels; andcompute a location for the apparatus based on the multi-channeltime-of-arrivals computed with respect to each base station, themulti-channel time-of-arrivals contained in the time-of-arrival reportmessages and known locations of the base stations.

From the perspective of a base station that is participating in theprocess described herein, a method is provided comprising: at a firstbase station operating in a wireless network, on each of multiplechannels performing ranging exchanges with each of a plurality of otherbase stations and storing samples of received ranging signals associatedwith the ranging exchanges; sending from the first base stationtime-of-departure report messages including information indicating:single-channel time-of-departures for ranging signals transmitted by thefirst base station on respective ones of the multiple channels, thechannel on which the ranging signals were each sent and the destinationbase station to which each ranging signal was sent; receivingtime-of-departure report messages from each of the plurality of otherbase stations; computing a multi-channel wireless channel responsesbetween the first base station and each of the plurality of other basestations based on the stored samples of received ranging signals on eachof the multiple channels and the time-of-departure report messagesreceived from the each of the plurality of other base stations;computing multi-channel time-of-arrivals between the base station andeach of the plurality of other base stations based on the multi-channelwireless channel responses; and sending time-of-arrival report messagesto each of the plurality of other base stations, each time-of-arrivalreport message including information representing multi-channeltime-of-arrivals from received ranging signals during ranging exchangeson the multiple channels.

The above description is intended by way of example only.

What is claimed is:
 1. A method comprising: at a wireless deviceoperating in a wireless network that includes a plurality of wirelessbase stations at known locations, receiving on each of multiplechannels, ranging signals associated with ranging exchanges between thebase stations and storing at the wireless device samples of receivedranging signals associated with the ranging exchanges; capturingtime-of-departure report messages transmitted between base stations,each time-of-departure report message including information indicating,for the base station that is the source of the time-of-departure reportmessage: single-channel time-of-departures for ranging signalstransmitted by the base station on respective ones of the multiplechannels on which the ranging signals were each sent and the destinationbase station to which each ranging signal was sent; computingmulti-channel wireless channel responses between the wireless device andeach of the base stations based on the stored samples of receivedranging signals at the wireless device on each of the multiple channelsand the time-of-departure report messages captured from the basestations; computing multi-channel time-of-arrivals between the wirelessdevice and each of the base stations based on the multi-channel wirelesschannel responses computed for the wireless device; capturingtime-of-arrival report messages transmitted between base stations, eachtime-of-arrival report message including information representingmulti-channel time-of-arrivals from received ranging signals duringranging exchanges on the multiple channels; and computing a location forthe wireless device based on the multi-channel time-of-arrivals computedfor the wireless device with respect to each base station, themulti-channel time-of-arrivals contained in the time-of-arrival reportmessages transmitted between base stations and the known locations ofthe base stations.
 2. The method of claim 1, wherein receiving comprisesreceiving ranging signals associated with ranging exchanges between basestations on one channel, waiting a period of time, and thereafterreceiving ranging signals associated with ranging exchanges between basestations on another channel.
 3. The method of claim 2, furthercomprising receiving at the wireless device from at least one of thebase stations a message indicating a schedule and identification of thechannels in which the ranging exchanges between base stations are to bemade.
 4. The method of claim 3, further comprising the wireless deviceswitching channels of a receiver in accordance with the schedule inorder to receive ranging signals associated with ranging exchangesbetween base stations on each of the multiple channels and capturetime-of-departure report messages and multi-channel time-of-arrivalreport messages transmitted between base stations.
 5. The method ofclaim 1, wherein the multiple channels contiguously span a frequencyband of interest.
 6. The method of claim 5, wherein the multiplechannels partially overlap.
 7. The method of claim 1, wherein computingthe multi-channel wireless channel responses comprises correcting forclock offsets between the base stations, and computing the location ofthe wireless device comprises correcting for clock offsets between thebase stations.
 8. The method of claim 1, wherein computing the locationcomprises: computing a predicted differential time of flight from afirst base station to a second base station with respect to the wirelessdevice; computing a measured differential time-of-arrival between thefirst base station and second base station; and deriving the location ofthe wireless device from a difference between the predicted differentialtime of flight and measured differential time of flight.
 9. The methodof claim 1, further comprising computing a full multiple-inputmultiple-output (MIMO) multi-channel wireless channel response withrespect to each of the plurality of base stations, computing an angle ofarrival for a MIMO wireless channel response at a first path of amultipath channel response, and wherein computing the location is basedfurther on the angle of arrival.
 10. An apparatus comprising: a receiverconfigured to receive wireless signals including signals transmittedbetween base stations as part of ranging exchanges between the basestations; a memory configured to store data derived from reception ofwireless signals by the receiver; and a controller coupled to thereceiver, wherein the controller is configured to: control the receiverto receive on each of multiple channels, ranging signals associated withranging exchanges between the base stations; store in the memory samplesof received ranging signals associated with the ranging exchanges;extract from received time-of-departure report messages transmittedbetween base stations, information indicating, for the base station thatis the source of the time-of-departure report message: single-channeltime-of-departures for ranging signals transmitted by the base stationon respective ones of the multiple channels the channel on which theranging signals were each sent and the destination base station to whicheach ranging signal was sent; compute multi-channel wireless channelresponses with respect to each of the base stations based on the storedsamples of received ranging signals on each of the multiple channels andthe time-of-departure report messages; compute multi-channeltime-of-arrivals with respect to each of the base stations based on themulti-channel wireless channel responses; extract from receivedtime-of-arrival report messages transmitted between base stationsinformation representing multi-channel time-of-arrivals from receivedranging signals during ranging exchanges on the multiple channels; andcompute a location for the apparatus based on the multi-channeltime-of-arrivals computed with respect to each base station, themulti-channel time-of-arrivals contained in the time-of-arrival reportmessages and known locations of the base stations.
 11. The apparatus ofclaim 10, wherein the controller is configured to control the receiverto switch channels based on a received message indicating a schedule andidentification of the channels in which the ranging exchanges betweenbase stations are to be made in order to receive ranging signalsassociated with ranging exchanges between base stations on each of themultiple channels and capture time-of-departure report messages andmulti-channel time-of-arrival report messages transmitted between basestations.
 12. The apparatus of claim 10, wherein the controller isconfigured to compute the multi-channel wireless channel responses bycorrecting for clock offsets between the base stations, and to computethe location by correcting for clock offsets between the base stations.13. The apparatus of claim 10, wherein the controller is configured tocontrol the receiver to switch to each of the multiple channels whichcontiguously span a frequency band of interest.
 14. The apparatus ofclaim 10, wherein the controller is configured to compute themulti-channel wireless channel responses by correcting for clock offsetsbetween base stations, and to compute the location by correcting forclock offsets between the base stations.
 15. The apparatus of claim 10,wherein the controller is configured to compute the location by:computing a predicted differential time of flight from a first basestation to a second base station; computing a measured differentialtime-of-arrival between the first base station and second base station;and deriving the location from a difference between the predicteddifferential time of flight and measured differential time of flight.16. A method comprising: at a first base station operating in a wirelessnetwork, on each of multiple channels performing ranging exchanges witheach of a plurality of other base stations and storing samples ofreceived ranging signals associated with the ranging exchanges; sendingfrom the first base station time-of-departure report messages includinginformation indicating: single-channel time-of-departures for rangingsignals transmitted by the first base station on respective ones of themultiple channels, the channel on which the ranging signals were eachsent and the destination base station to which each ranging signal wassent; receiving time-of-departure report messages from each of theplurality of other base stations; computing a multi-channel wirelesschannel responses between the first base station and each of theplurality of other base stations based on the stored samples of receivedranging signals on each of the multiple channels and thetime-of-departure report messages received from the each of theplurality of other base stations; computing multi-channeltime-of-arrivals between the base station and each of the plurality ofother base stations based on the multi-channel wireless channelresponses; and sending time-of-arrival report messages to each of theplurality of other base stations, each time-of-arrival report messageincluding information representing multi-channel time-of-arrivals fromreceived ranging signals during ranging exchanges on the multiplechannels.
 17. The method of claim 16, wherein performing rangingexchanges comprises sending a signal from the first base station to eachof the plurality of other base stations and receiving a response fromeach of the other base stations, and receiving a signal from each of theother base stations and sending a response to each of the other basestations.
 18. The method of claim 17, further comprising performingranging exchanges on one channel, waiting a period of time, andthereafter performing ranging exchanges on another channel.
 19. Themethod of claim 17, wherein the multiple channels contiguously span afrequency band of interest.
 20. The method of claim 17, whereincomputing the multi-channel wireless channel responses comprisescorrecting for clock offsets between the first base station and each ofthe plurality of other base stations.